1. Field of the Invention
This invention relates in general to electrical cable, and in particular to an electrical cable for use with submersible pumps.
2. Description of the Prior Art
This invention concerns an electrical power cable used to power a downhole electrical motor for a submersible pump. These submersible pumps normally pump a mixture of oil and brine from wells often several thousand feet deep and often under high temperatures. The electrical cable normally consists of three stranded conductors. Each stranded conductor contains an insulating layer of a material that is resistant to oil and brine. Typically, an elastomeric jacket is extruded over the three conductors and an outer metallic armor surrounds the jacket.
In wells that have a significant gas content, gas permeation of the jacket occurs by way of absorption or defects. Periodically, all submersible pumps must be pulled to the surface for servicing. As the pump is pulled to the surface, the pressure and temperature both rapidly decrease. If gas has permeated the jacket, the reduction in temperature and pressure traps low molecular weight gasses in the cable. The basically, non-porous impermeable jacket does not allow the gas to escape rapidly. The gas within expands under reduced pressure, causing the jacket to balloon, and rupture.
An improved structure is disclosed in U.S. Pat. Nos. 4,088,830, issued May 9,1978 and U.S. Pat. No. 4,096,351, issued June 20, 1978, the inventors of both of which are Robert V. Wargin an Clinton A. Boyd. These patents teach the use of an insulating layer of thermosetting material that is resistant to oil and brine, but does allow some absorption of gas. The insulating material is relatively thin and allows gas to rapidly desorb when the cable is being pulled to the surface. A fiber braid surrounds each conductor, and contains the porous insulation layer to prevent rupturing of the insulation layer during depressurization. In the '351 patent, the conductors are surrounded by metallic armor, and in the '830 patent, the conductors are surrounded by a polypropolene, perforated layer, which serves as the armor. While the cables of these two patents perform successfully, the braid surrounding each conductor individually adds considerably to the cost of the cable.